The present invention relates to an image forming apparatus such as a printer, a facsimile and a copying machine for forming an image on recording material such as paper using electrophotography. Particularly, the present invention relates to technique for controlling transfer voltage in an image forming apparatus for forming an image on both sides of recording material.
In addition, the present invention relates to a color image forming apparatus such as a printer, a facsimile and a copying machine for forming a color image on recording material such as paper using electrophotography. Particularly, the present invention relates to transfer technique for transferring a toner image on recording material.
Generally, an image forming apparatus for forming an image on recording material such as paper using electrophotography is provided with an image carrier on the surface of which a toner image is formed, transfer member for forming a transfer part between the transfer member and the image carrier, applying transfer voltage and transferring the toner image on the surface of the image carrier on the recording material and fixing member for heating the recording material, passing the recording material on which the toner image is transferred by the transfer member and fixing the toner image on the recording material.
In such an apparatus, as the resistance value of recording material also differs when the hygroscopic degree of the recording material such as paper differs by the change of ambient temperature and humidity, the suitable value of transfer voltage when a toner image is transferred on the recording material also differs. In this connection, there is known an apparatus provided with humidity detector to determining the optimum value of transfer voltage when a toner image is transferred on the recording material according to humidity.
And there is also known an image forming apparatus, disclosed in Unexamined Japanese Patent Publication No. 2-285377A, in which transfer voltage is controlled under constant current with the minimum voltage and the maximum voltage limited so as to always enable stable transfer in any environment independent of the change of the characteristic of transfer material and the type of transfer material.
Besides, as the recording material is normally held with it laminated in a paper feeder and others and such a paper feeder and others are constituted so that the uppermost recording material is first fed, the uppermost recording material is most influenced by humidity (the hygroscopic degree is most increased).
Heretofore, the above point is noticed and there is proposed a technique, disclosed in Japanese Patent Publication No. 7-082271B, which enables detecting humidity by a humidity detector, measuring an interval between each recording, that is, between each fed recording material (for example, time until the next recording material is fed since the uppermost recording material of laminated recording material is fed) and determining transfer voltage based upon the above humidity and the above interval.
Recently, a demand for forming an image not only on one side of recording material but on both sides increases.
The formation of an image on both sides of recording material can be realized by transferring a toner image on the first side (for example, the right side) in a transfer part, reversing the two sides of recording material on which the toner image is fixed after the recording material passes fixing member, returning the recording material to the transfer part again, also transferring and fixing a toner image on the second side (for example, the back side).
However, it is found that if a toner image is formed on both sides of recording material, the optimum value of transfer voltage in transferring on the first side and the optimum value of transfer voltage in transferring on the second side are greatly different.
Though recording material does not pass fixing member yet in transferring on the first side, the recording material already passes the fixing member once in transferring on the second side and as fixing member heats recording material and fixes a toner image on the recording material, not only the hygroscopic degree of recording material immediately after the recording material passes fixing member approaches zero substantially but the recording material is in a state extremely sensitively influenced by humidity.
In such a situation, the related art disclosed in the above Japanese Patent Publication No. 7-082271B does not function effectively. Because the above related art can function only in case an image is formed only on one side.
Recently, a demand for forming a color image using toner in plural colors increases. Such a color image can be formed by superposing toner images in plural colors. In such a case, as toner images in plural colors are required to be transferred when a color image is transferred, a range of the suitable values of the above transfer voltage is small.
As in Japanese Patent Publication No. 7-050361B, there is proposed a color image forming apparatus in which recording material such as paper is wound and held on a drum and toner images in plural colors are directly and sequentially superposed on the recording material is proposed. In the color image forming apparatus, three areas divided based upon ambient temperature and humidity, that is, a low-temperature and low-humidity environmental area (I), a normal-temperature and normal humidity environmental area (II) and a high-temperature and high-humidity environmental area (III) are set, an optimum transfer current value is set every area in a table beforehand, a signal from detector for detecting temperature and humidity is converted from analog to digital, an optimum transfer current value in the above table is selected based upon the output, and transfer is executed based upon the transfer current value.
As the above color image forming apparatus disclosed in Japanese Patent Publication No. 7-050361B is constituted so that recording material such as paper is wound and held on the drum and toner images in plural colors are directly and sequentially superposed on the recording material, there is a drawback that a member for winding and holding recording material on the drum and a member for releasing winding and separating the recording material from the drum are required and thereby the constitution is complicated.
The above drawback can be solved by using constitution that toner images in plural colors are sequentially superposed on the surface of an image carrier and the superposed toner images in plural colors are collectively transferred on recording material. According to the above constitution, recording material is not required to be wound and held on a drum and therefore, winding is also not required to be released.
However, in the case of such constitution, that is, if toner images in plural colors superposed on the surface of an image carrier are collectively transferred on recording material, it is found that a satisfactory transfer condition may not necessarily be obtained depending upon the above any related arts. That is, it is found that a satisfactory transfer condition may not be obtained only by controlling transfer voltage under constant current with the minimum voltage and the maximum voltage limited and a satisfactory transfer condition may not be obtained only by setting an optimum transfer current value beforehand every area divided based upon temperature and humidity and selecting a transfer current value based upon temperature and humidity.
It is also found that as superposed toner images in plural colors were collectively transferred if toner images in plural colors superposed on the surface of an image carrier were collectively transferred on recording material, a range of transfer conditions under which a satisfactory transfer condition can be obtained was very small. Therefore, the inventors also found that it was influenced not only by environment and the type of recording material but by the dispersion of the resistance of a member forming a path through which transfer voltage was supplied whether a transfer condition was satisfactory or not.
According to the above related art, as there is naturally a limit in the precision of the detector and A/D conversion, an optimum transfer current value according to temperature and humidity may not necessarily be selected if the temperature and/or humidity are/is in the vicinity of a boundary between areas. For example, if temperature and humidity are in the vicinity of a boundary with the area (I) though actual temperature and humidity are in the area (II), an optimum transfer current value corresponding to the area (I) may be selected and if the above temperature and humidity are in the vicinity of a boundary with the area (III), an optimum transfer current corresponding to the area (III) may be selected. In such cases, as transfer is executed using a current value different from an optimum transfer current value to be selected originally, transfer failure occurs.
For a method of improving such a situation, the following methods are conceivable:
(1) A method of increasing the number of bits converted from analog to digital and the capacity of the table.
For example, temperature and humidity are expressed by each 8-bit and a table with capacity corresponding to 256.times.256 types of data is generated.
(2) A method of executing A/D conversion with high resolution (for example, 8-bit) and generating a table corresponding to only data for a high order few bits.
For example, if a table corresponding to only data for high order 3-bit is generated, a table with capacity corresponding to 8.times.8 types of data has only to be generated.
(3) A method of processing temperature and humidity data converted from analog to digital by operation without referring to a table and determining a transfer current value.
However, as the capacity of the table is too large according to the method (1), the method (1) is not desirable.
As 8-bit are only simply converted to 3-bit for example according to the method (2), an error of detection is increased in a nonlinear system.
As complicated calculation is required in a nonlinear system according to the method (3), table conversion is sometimes used in the process of the calculation in the end and is not efficient.